1. Field of the Invention
The present invention relates to a die crowning apparatus for a press brake and more specifically to an apparatus for adjusting the curvature of a die so that a pair of upper and lower dies are parallel to each other all over the length of the press brake during workpiece bending process. Here, "Crowning" implies to form something into an upwardly convex state and "press brake" implies a press for bending a long workpiece at an angle.
2. Description of the Prior Art
As is well known, a press brake is provided with an upper table for supporting an upper die and a lower table for supporting a lower die so as to face each other, both ends of each of the upper and lower tables being supported by two side frames. A sheet (workpiece) disposed between the two dies is bent when any one of the upper and lower tables (ram) is moved up and down by a drive unit via a crankshaft, hydraulic cylinder, etc. so as to engage the two dies with each other.
In the above bending processing of a sheet-like workpiece, it is well known that the upper and lower tables are deformed or curved in the vertical direction. This is because the two tables are supported at both the extreme ends thereof by the two frames; a drive unit for moving one of the tables up and down is provided on either side; and there exists a reaction of a sheet workpiece generated when the sheet is being bent. That is, the upper table tends to deform upward at the middle along the longitudinal direction of the brake press and the lower table tends to deform downward also at the middle therealong. In case the upper and lower tables are deformed or curved, there arises a problem such that the two upper and lower dies are not engaged with each other accurately near the middle of the tables and therefore a bending angle at the middle of a sheet to be bent is different (e.g. bending angle is large) from that at both the ends of the sheet.
To overcome the above-mentioned problem, various methods or apparatus have been so far proposed to obtain a uniform bending angle all over a long sheet-like workpiece by engaging the two dies in parallel with each other all over the dies.
For instance, a plurality of wedge members are arranged between the upper table and the upper die or between the lower table and the lower die. In this first example, each of the wedge members should be adjusted in position independently to obtain an appropriate crown or convex state of the die, thus resulting in a complicated adjustment work and a low workability.
Further, a plurality of wedge members are engaged with a common slidable member so that all the wedge members can be adjusted in position by moving the slidable member. Or else, a plurality of wedge adjust screws are associated with the wedge members, respectively so that all the wedge member can be adjusted by rotating all the adjust screws simultaneously. In this second example, it is necessary to prepare different kinds of wedges having different slope angles respectively, or to adjust screws having different pitches, thus resulting in complicated manufacturing processes.
Furthermore, a plurality of minicylinders are provided horizontally for the upper table or the lower table to bend the middle portion of the table upwardly. In this third example, a number of minicylinders and pressure regulation valves are required, thus resulting in a higher manufacturing cost and a complicated cylinder pressure setting work.